In flat projection radiography examination types exist that aim at imaging a larger portion of the body than can be fit on a single, even the largest imaging cassette.
Such imaging needs arise in so-called full-leg or full-spine examinations, where clinical indication requires that e.g. the full spine or the full leg is examined at once so as to enable or to quantify diagnosis.
Analogous to conventional film-screen based radiography, in storage phosphor image acquisition wherein a radiation image is temporarily stored in a photostimulable phosphor screen, such examinations are achieved by filling a larger cassette with a plurality of storage phosphor screens, generally overlapping with one another so as to completely cover the longer, elongate cassette (an embodiment referred to as an overlapping sheet embodiment).
Such elongate cassettes are typically 35.times.105 cm for full-leg examinations and 30.times.90 cm for full-spine examinations. In the overlapping sheet embodiment, the largest single film cassette measures 35.times.43 cm and hence is unsuitable for imaging the spinal column or a leg. Typically, 4 screens are used, e.g. 4 (24.times.30 cm) screens are arranged in a 30.times.90 cm cassette, clearly resulting in an overlap configuration. Therefore, the part of the image corresponding to the overlap zone, will be less exposed.
Alternatively, the situation of non-overlapping images may also occur, e.g. in a configuration of 3 (35.times.35 cm) imaging screens in a 35.times.105 cm cassette, resulting in 3 pair-wise touching but otherwise non-overlapping images.
During exposure, a fixed grid is simultaneously present in the path of the X-ray beam, resulting in an image of a raster of horizontal and vertical parallel thin lines superposed to the radiation image of the elongate body. These lines aid the radiologist or operator in reconstructing the original geometry of the body, since lines on image must necessarily extend continuously and seamlessly into lines of the previous image and the next image.
In U.S. Ser. No. 09/035,528, a method is disclosed to acquire a radiation image of an elongate body by using a sequence of recording members in partially overlapping disposition.
This patent application further relates to a method to obtain a single `stitched` image of an elongate body using the images read out of each of these recording members.
The image acquisition process assumes the use of an elongate cassette in which a sequence of stimulable phosphor screens are arranged in an overlapping manner so as to completely cover the cassette.
After exposure, the elongate cassette is opened, the individual screens are removed from the cassette and are each put into a smaller cassette of a size corresponding to the dimensions of an individual screen. Then, the cassettes are sequentially fed into a dedicated read-out apparatus where the cassette is again opened, the screen is taken out of the cassette, and scanned by means of stimulating radiation. The image-wise modulated light that is emitted upon stimulation is detected and converted into a digital image representation of part of the elongate body. Next, the digital image representations read out of each of the cassettes are recombined to form an image of the entire elongate body.
The handling process has several drawbacks.
First, a dark room is needed to transfer the individual screens from the elongate cassette to their corresponding individual regular size cassettes, since ambient light impinging on a screen may stimulate it, and hence may (at least partially) erase its contents.
Second, the operator handling the screens may accidentally damage them, e.g. by scratching them.
Third, the overall time-to-diagnosis is lengthened due to the additional operations and potentially increased risk of re-take.
U.S. patent application Ser. No. 09/035,528 further discloses a method of reconstructing the image of the elongate body from the partial images read out from each of the overlapping recording members, each having stored therein a part of the elongate scene. The distortions coped with in aforementioned patent application were limited to shift, overlap and rotation of each sub-image with respect to the previous and next sub-image. The method disclosed in above patent application depends on two basic algorithms: (1) detection and modelling of the periodic grid and (2) cross-correlation of overlapping image parts. The periodic grid is important to be able to align and stitch the images with respect to each other. The grid is located physically on the elongate cassette and covers its full surface in a continuous way. Hence, the grid should also be present in the resulting re-constructed image composition in a continuous way.